When patterns are produced on a sheet or other substrate moving relatively to a highly directive nozzle from which a printing liquid is sprayed upon that sheet at close range, a precise timing of the spray is important for bringing out the fine details of the pattern. Control pulses conforming to the distribution of a particular color in the pattern can be readily generated with the aid of a suitable programmer, yet the mechanical and electrical inertia of the valve and its actuating coil tends to introduce distortions. Whereas the mechanical inertia can be minimized by improvements in valve design as disclosed in the application referred to, the electrical inertia due to the inductance of the actuating coil remains a separate problem.